Xianshu Wang

Enhancing electrochemical performance of Li/LiMn 2 O 4 cell at elevated temperature by tailoring cathode interface via diethyl phenylphosphonite (DEPP) incorporation

Diethyl phenylphosphonite (DEPP) is used as a novel electrolyte additive to improve the cyclability of spinel LiMn2O4 upon cycling at elevated temperature (55xa0°C). The charge/discharge cycling stability results indicate that capacity retention of Li/LiMn2O4 cell is significantly improved from 40 to 78% at a rate of 1C (1Cxa0=xa0120xa0mAhxa0g−1) when 1.0 wt% DEPP is added to the baseline electrolyte (1.0xa0M LiPF6 in EC/EMC/DEC (3:5:2, vol.%)) after 450 cycles at 55xa0°C. This improvement can be attributed to the preferential oxidation of DEPP to that of the baseline electrolyte and the subsequent formation of a protective film on the cathode surface. This passivation film suppresses detrimental electrolyte decomposition and in turn protects LiMn2O4 from further decomposition. Molecular energy level calculations, linear sweep voltammetry, and cyclic voltammetry results confirm that DEPP is oxidized on the cathode surface prior to the oxidation of carbonate solvents. Electrochemical impedance spectroscopy illustrates that the the cathode interfacial film generated from DEPP oxidation is more stable and robust than that of the surface film yielded from the baseline electrolyte’s decomposition. Ex situ surface-characterization results further support the claim that DEPP incorporation in the electrolyte suppresses the electrolyte oxidation at elevated temperature and the decomposition of LiMn2O4 cathode material as well.Graphical AbstractThe highest occupied molecular orbital energy of DEPP (−6.75305xa0eV) is higher than that of EC (−8.73078xa0eV), EMC (−8.40479xa0eV), and DEC (−8.36043xa0eV). Furthermore, the oxidation potential and the ionization energy also demonstrate that DEPP is more easily oxidized than electrolyte solvents. Therefore, DEPP is preferred to be oxidized on the cathode than the baseline electrolyte.

Significantly improved cyclability of lithium manganese oxide, simultaneously inhibiting electrochemical and thermal decomposition of the electrolyte by the use of an additive

Lithium manganese oxide (LiMn2O4) is one of the most promising cathodes for lithium ion batteries because of its abundant resources and easy preparation. However, its poor cyclability, especially under elevated temperature, limits its application on a large scale. In this work, it is reported that the cyclability of LiMn2O4 can be significantly improved by applying 4-(trifluoromethyl)benzonitrile (4-TB) as an electrolyte additive. Charge/discharge tests indicate that the capacity retention of LiMn2O4 after 450 cycles at 1C and 55xa0°C in a standard electrolyte, 1 M LiPF6 in EC/EMC/DEC (3u2006:u20065u2006:u20062, in weight), is improved from 19% to 69%. Further electrochemical and physical characterization demonstrates that 4-TB can, on the one hand, be electrochemically oxidized preferentially compared to the standard electrolyte, which generates a protective interphase film on LiMn2O4. On the other hand, 4-TB can effectively combine with protonic impurities, which inhibits the thermal decomposition of the electrolyte. This dual-functionality of 4-TB contributes to the significantly improved cyclability of LiMn2O4.

Functionalized N-doped hollow carbon spheres as sulfur host with enhanced electrochemical performances of lithium-sulfur batteries

A novel composite, functionalized N-doped hollow carbon spheres (F-NHCS), is proposed as cathode host for improving the cycle stability and rate capability of a Li-S battery, which involves the functionalization of NHCS with oxygen-containing groups. The high electronic conductivity and abundant oxygen-containing functional groups of F-NHCS can not only be served as the conductive matrix for loading S as the cathode (F-NHCS-S) but also exhibit strong absorption capability to anchor S atoms and thus effectively solve the dissolution issue of intermediates polysulfides during cycling. As a result, the as-obtained F-NHCS-S cathode exhibits excellent performances: delivering an initial specific capacity of 827xa0mAhxa0g−1 at 0.2xa0C with about 74xa0wt% S loading and retaining a specific capacity of 549xa0mAhxa0g−1 after 100xa0cycles, compared to the respective 747xa0mAhxa0g−1 and 182xa0mAhxa0g−1 of the cathode based on the host without N-doping and functionalization (HCS-S). The fabrications of the hosts and the corresponding cathodes are demonstrated by the physical characterizations from SEM, TEM, XPS, XRD, FT-IR, BET, and TGA.